Self-assembled monolayer and method of making

ABSTRACT

According to the present invention, the previously known functional material having a self-assembled monolayer on a substrate has a plurality of assembly molecules each with an assembly atom with a plurality of bonding sites (four sites when silicon is the assembly molecule) wherein a bonding fraction (or fraction) of fully bonded assembly atoms (the plurality of bonding sites bonded to an oxygen atom) has a maximum when made by liquid solution deposition, for example a maximum of 40% when silicon is the assembly molecule, and maximum surface density of assembly molecules was 5 silanes per square nanometer. Note that bonding fraction and surface population are independent parameters. The method of the present invention is an improvement to the known method for making a siloxane layer on a substrate, wherein instead of a liquid phase solution chemistry, the improvement is a supercritical phase chemistry. The present invention has the advantages of greater fraction of oxygen bonds, greater surface density of assembly molecules and reduced time for reaction of about 5 minutes to about 24 hours.

CROSS REFERENCE TO RELATED INVENTION

This application is a Continuation-In-Part of application Ser. No.09/272,762, filed Mar. 19, 1999, now abandoned.

This invention was made with Government support under ContractDE-AC0676RLO1830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention is a self-assembled monolayer and method ofmaking.

BACKGROUND OF THE INVENTION

Since their unveiling in 1992, mesoporous ceramics have inspiredsubstantial interest, especially by adding self-assembling monolayercompounds to the surface(s) of the mesopores. By varying the terminalgroup of the self-assembling monolayer, various chemicallyfunctionalized materials have been prepared. A mesoporous material isdefined as a material, usually catalytic material, having pores with adiameter or width range of 2 nanometers to 0.05 micrometers.

Exemplary of use of self-assembling monolayer(s) on a mesoporousmaterial is the International Application Publication WO 98/34723(E-1479 CIP PCT). The self-assembling monolayer(s) is made up of aplurality of assembly molecules each having an attaching group. Forattaching to silica, the attaching group may include a silicon atom withas many as four attachment sites, for example; siloxanes, silazanes, andchlorosilanes. Alternative attaching groups include metal phosphate,hydroxamic acid, carboxylate, thiol, amine and combinations thereof forattaching to a metal oxide; thiol, amine, and combinations thereof forattaching to a metal; and chlorosilane for attaching to a polymer. Acarbon chain spacer or linker extends from the attaching group and has afunctional group attached to the end opposite the attaching group.

Methods of attaching and constructing the self-assembling monolayer on amesoporous material involve solution deposition chemistry in thepresence of water. More specifically, as reported by Feng, X.; Fryxell,G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. Science, 1997, 276,923-926 (Feng et al, 1997); and Liu, J.; Feng, X.; Fryxell, G. E.; Wang,L. Q.; Kim, A. Y.; Gong, M. Adv. Mat. 1998, 10, 161-165 (Liu et al.,1998), a synthetic protocol to prepare monolayers of MPTMS(mercaptopropyl trimethoxysilane) within the pores of MCM-41 involved a1-hour hydration step, followed by a 6-hour silanation step in refluxingtoluene. At this stage, the silane coverage is limited to approximately3.6-4.0 silane molecules/nm² (this surface density is not enhanced byeither extending the reaction time or increasing silane concentration).Following the silanation with a 2-3 hour azeotropic distillation drivesthe equilibria through the removal of reaction by-products, andincreases this surface density to 5.0-5.2 silanes/nm². This surfacedensity is representative of typical silane-based monolayers. Themonolayer coated mesoporous product is then isolated by filtration,washed extensively and then dried for several days. In summary, theoverall procedure takes about 10 hours of laboratory prep time and 1-10days of drying time. The time is driven by the kinetics of getting theself-assembling molecules into the mesopores and getting the water andany other solvent out of the mesopores.

The product obtained exhibits a maximum of 40% of the monolayer siliconatoms fully crosslinked for maximizing monlayer stability. Ideally, 100%of the silicon atoms would be fully crosslinked. Full crosslinking ishaving three of the four bonding sites linked to another silicon atomvia an oxygen atom, with the fourth linked to the functional groupterminated hydrocarbon chain. However, the presence of “dangling”hydroxyl groups (OH—) cannot be avoided in the solution method and it isthe presence of the “dangling” hydroxyl groups that interferes with thecrosslinking of the monolayer, thus placing a practical upper limit onthe number of silicon atoms that are fully crosslinked of 40%.

Thermal “curing” of silane monolayers, wherein typical thermal curing(ca. 150° C.), of a silane monolayer creates a terminal to internalsilane ratio of 1:2 corresponding to about 60% to 65% of attachingmolecules (silicon) fully crosslinked.

Hence, there remains a need for a mesoporous material havingself-assembling monolayer thereon with a greater fraction of theassembly atoms fully crosslinked. There is also a need for greatersurface density of silicon atoms, which may also be expressed as agreater surface density of monolayer coverage. Finally, there is a needfor a method of making these materials that is less time consuming.

The main difficulty in functionalizing microporous materials may beattributed to diffusion of the organic molecules intoto the small porechannels. In the last few years, both post-silanization and in-situdeposition have been successfully applied to mesoporous materials, inwhich the pore diameter is usually larger than 2 nm. The mesoporousmaterials (usually synthesized using surfactant micelles as templates)have very uniform pore sizes. Because of their high surface area and theopen pore channels; functionalized mesoporous materials have beeninvestigated for many adsorption and catalysis applications. However dueto the large pore size and the amorphous nature of the materials, thesematerials are not likely to find application as size selectivecatalysts.

A zeolite is any one of a family of hydrous aluminum silicate minerals,whose molecules enclose cations of sodium, potassium, calcium,strontium, or barium, or a corresponding synthetic compound, usedchiefly as molecular filters and ion-exchange agents. Compared to themesoporous materials, the diffusion of organic molecules in zeolites isseverely hindered by the small pore size. Deposition of silanes on theexterior surface is therefore greatly favored over silanation ofinternal surfaces. Heretofore, it had been believed that introducingorganic functional groups to the internal pore surfaces of commercialzeolites to produce size selective microporous catalysts could not beachieved due to the size of the pores.

SUMMARY OF THE INVENTION

According to the present invention, the previously known functionalmaterial having a self-assembled monolayer on a substrate has aplurality of assembly molecules each with an assembly atom with aplurality of bonding sites wherein a bonding fraction (or fraction) offully bonded assembly atoms (fully crosslinked assembly atoms) with theplurality of bonding sites (the plurality of bonding sites bonded to anoxygen atom) exceeds a maximum compared to solution deposition, andmaximum surface density of assembly molecules greater than for solutiondeposition. For example, with the assembly atom silicon, having 4bonding sites, the bonding fraction maximum for solution deposition was40% as deposited or about 60% to 65% (a terminal to internal silaneratio of about 1:2) after thermal curing, and maximum surface density ofsilane molecules was 5.2 silanes per square nanometer. Note thatcrosslinking fraction and surface density are separate parameters.

The method of the present invention is an improvement to the knownmethod for making a self-assembled monolayer on a substrate, whereininstead of a liquid phase solution chemistry, the improvement is asupercritical phase chemistry.

The present invention has the advantages of greater fraction of bridgingoxygen bonds, and greater surface density of assembly moleculesresulting in a lower defect coating that enhances thermal and chemicalstability or resistance. Further, hydrolysis and deposition is completewithin 5 minutes, a surprising rate enhancement of more than two ordersof magnitude. Not only are the hydrolysis and deposition considerablyaccelerated relative to standard solution methods, but the final dryingphase has been completely eliminated by the use of a supercritical fluidas the reaction medium. The product emerges from the reaction chamberdry and ready to use. This represents considerable timesavings.

Water is a necessary reactant in the hydrolysis and condensationchemistry of alkylsilanes to form self-assembled monolayers onto ceramicoxide surfaces. It must be present in appropriate (stoichiometric)amounts; too little will result in incomplete deposition andcrosslinking and too much will result in bulk solution phase polymerformation. Experience has shown that approximately 10¹³ water moleculesper square meter of available surface area is optimum. This amounts toapproximately 2 water molecules for each silane to be anchored.

It is also important that this water be intimately associated with thesurface and not free in solution. By having the water in close proximityto the ceramic oxide surface, the silane hydrolysis/condensationchemistry can only take place on the surface, thereby favoring thedesired monolayer deposition and avoiding solution phase polymerization(which leads to bulk amorphous polymer and blocked pores). Thisassociation is necessary to obtain any thin film morphology, and iscritical to obtain clean monolayer formation.

In addition, the water associated with the ceramic oxide surface must beevenly spread out across the surface. This causes the hydrolysischemistry to be uniformly spread out across the ceramic oxide surface,which reduces monolayer defect formation, while at the same timeminimizing bulk polymerization.

By adding the water first, and allowing it to fully equilibrate with theceramic oxide surface, Applicants are able to exploit the naturalaffinity that these ceramic oxides have for water and are thus able toinsure that these important conditions are met.

Adding water separately to a solution of silanes will result in bulksolution phase polymerization competing with any possible monolayerdeposition. This is counter-productive since it significantly depletesthe amount of silane available to form the monolayer, and in the case ofa mesoporous substrate, the bulk amorphous polymer will plug and blockthe pore channels, reducing the available surface area and restrictinginterfacial access, thus eliminating the most desirable features of sucha material.

In the liquid solution deposition of the prior art, a wastestream isproduced as a mixture of water, methanol, toluene and small amounts ofmercaptan that failed to be deposited. It is impractical to separatethis mixture, and therefore the mixture is usually disposed of ashazardous waste. According to the present invention using asupercritical fluid for solution deposition, the only by-product of thereaction (hydrolysis) is an alcohol (e.g. methanol), which is easilyseparated from the supercritical fluid (which can be recovered forrecycling). In fact, the alcohol is of sufficient purity to represent apotential feedstock that can be sold or recycled.

A further advantage of using a supercritical fluid as the reactionmedium is the elimination of flammable solvents, and performing thereaction under completely non-flammable conditions, which can be asignificant concern upon scale-up.

The subject matter of the present invention is particularly pointed outand distinctly claimed in the concluding portion of this specification.However, both the organization and method of operation, together withfurther advantages and objects thereof, may best be understood byreference to the following description taken in connection withaccompanying drawings wherein like reference characters refer to likeelements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of percent composition versus time for thehydroxylated silanol.

FIG. 2a is an NMR spectrum of a 5-minute sample made according to thepresent invention.

FIG. 2b is an NMR spectrum of a 24-hour sample made according to thepresent invention.

FIG. 2c is a peak ratio (percent) versus time for various samples.

FIG. 3 is an NMR spectrum of a 5-minute sample re-annealed for 30minutes.

FIG. 4 is a graph of sulfur concentration versus pH for samples madeaccording to the present invention and compared to samples made bysolution deposition exposed to corrosive solutions.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

According to the present invention, the previously known functionalmaterial having a self-assembled monolayer on a substrate, theself-assembled monolayer having a plurality of assembly molecules eachwith an assembly atom with a plurality of bonding sites wherein aportion of the assembly atoms have the plurality of bonding sites bondedto an oxygen atom is improved so that a greater portion of the assemblyatoms have the plurality of bonding sites bonded to an oxygen atom.

In addition, the known functional material having a self-assembledmonolayer on a substrate having a surface density by liquid solutiondeposition of the self-assembled molecules is improved so that thesurface density is greater.

The method of the present invention is an improvement to the knownmethod for making a self-assembled monolayer on a substrate, theself-assembled monolayer having a plurality of assembly molecules eachwith an assembly atom with a plurality of bonding sites, the methodhaving the step of bonding a plurality of oxygen atoms to a fraction ofthe plurality of bonding sites; wherein the improvement is the bondingdone by preparing the self-assembled monolayer in a supercritical fluid.

The supercritical fluid may be polar or non-polar. Compounds useful inthe supercritical phase according to the present invention includedcarbon dioxide, and possibly freons, nitrogen, noble gases, alkanes,alkenes, alkynes, and combinations thereof.

The thermal curing of the self-assembled monolayer may be during orafter forming of the self-assembled monolayer. In other words, aself-assembled monolayer that has been prepared in a supercritical fluidexhibiting a portion of assembly atoms fully crosslinked to oxygen atomsand a maximum surface density of assembly atoms may be exposed to asupercritical fluid for a time that is effective in convertinginterfering hydroxyl groups to bridging oxygen bonds, thereby increasingthe number of fully crosslinked silicon atoms to a greater portion.Alternatively, a self-assembled monolayer prepared by any other method,for example liquid solution chemistry, may be treated by adding assemblymolecules to the gaps of the alternatively prepared monolayer. Theassembly molecules are added in a supercritical fluid containingadditional assembly molecules. The greater portion is at least about75%, preferably greater than or equal to 80%.

The assembly atoms are selected to be compatible with the substrate. Forattaching to silica, the assembly atom may include a silicon atom withfour attachment sites, for example siloxane, silazane, and chlorosilane.Alternative assembly molecules include metal phosphate, hydroxamic acid,carboxylate, thiol, amine and combinations thereof for attaching to ametal oxide; thiol, amine, and combinations thereof for attaching to ametal; and chlorosilane for attaching to a polymer. A carbon chainspacer or linker may extend from the assembly atom and has a functionalgroup attached to the end opposite the assembly atom.

For silicon atoms having four bonding sites, the portion of fullycrosslinked bonding sites by supercritical fluid solution exposureincluding supercritical fluid solution deposition is greater than orequal to about 40% as deposited. Additional exposure time increases thefully crosslinked fraction to at least about 55%. Table 1 shows theamount of time for the percent of fully crosslinked siloxanes forsupercritical fluid processing at 7500 psi and 150° C. The times andpercent of full crosslinking are pressure and temperature dependent.

TABLE 1 Supercritical CO₂ Exposure Time for Percent of Fully CrosslinkedSiloxanes Time (hours) % of Fully Crosslinked Siloxanes ≦{fraction(1/12)} >40 4 55 24 75

In addition, supercritical fluid solution deposition results in greatersurface density of the assembly molecules. For siloxane, the surfacedensity is greater than 5.2 siloxane molecules per square nanometer, andhas been demonstrated up to 6.5 siloxane molecules per square nanometer.

Alternatively, or additionally, the surface deposition of theself-assembling monolayer(s) may be done in a manner of placing one ormore self-assembling monolayer precursor(s), including but not limitedto alkoxysilane, silazane, chlorosilane, and combinations thereof,together with mesoporous material that may be ceramic, for example metaloxide, including but not limited to silica, alumina, titania, andcombinations thereof, in a vessel that is subsequently filled with asupercritical fluid, including but not limited to carbon dioxide (CO₂),ethane (C₂H₆), ammonia (NH₃), and combinations thereof, to obtain theself-assembling monolayer(s) on the mesoporous or zeolite material. Byusing a supercritical fluid for deposition of the self-assemblingmonolayer, the surface density of silanes may be greater than 5.2silanes per square nanometer. The surface density is controlled by theamount of assembly molecule (e.g. silane) used for a given surface areaof mesoporous material. Moreover, deposition is complete in about 5minutes and no subsequent drying is needed. With a 5-minute deposition,the percent of fully crosslinked silion atoms is about 40%. Additionalsupercritical fluid exposure time increases the percentage of fullycrosslinked silicon atoms (see Table 1 above).

Further, the placing of the calcined mesoporous material may includemixing a sol-gel solution and surfactant for producing a mesoporousgreen body; removing the surfactant with the supercritical fluid andmaking a dry green body; and calcining said dry green body into theclaimed mesoporous material. In this manner, the entire process fromsol-gel templating through self-assembling monolayer deposition toincreasing the fraction of fully bonded silicon atoms may be done in asingle vessel in a supercritical fluid environment.

EXAMPLE 1

An experiment was conducted to test the influence of supercriticalcarbon dioxide (SCCO₂) on the hydration of a mesoporous silicadesignated MCM41, obtained by making the MCM-41 according to U.S. Pat.No. 5,264,203 (Mobil Oil Corporation, Fairfax, Va.). The calcinedsubstrate (primarily Q4 [non-hydroxylated silanol]) was free of anysilane(s).

Water was introduced to the pores of the MCM-41 sample via passivehydration in a 100% humidity chamber, followed by subjecting thehydrated sample to SCCO₂ forced hydrolysis at 100° C. This hydrationprotocol involved neutral pH, no salt, no ceramic oxide or organiccontaminants; just water, carbon dioxide and heat.

NMR analysis showed that this hydrolysis treatment was found to increasethe bonding fraction (hydroxylated silicon atoms) to 46% Q3 (surfacesilanol) and 8% Q2 (geminal silanol). This hydrolysis was carried out inthe presence of excess water and hydrolysis stopped at this point, withno damage to the mesostructure. The mild conditions of this hydrolysisprevent the dissolution of the MCM-41 since silicic acid is insoluble inSCCO₂, and thus there was very little risk of collapsing themesostructure. (SCCO₂ is very nonpolar, and approximates hexane in itssolvating power).

The hydrolysis reaction was complete in about 20 minutes (See FIG. 1).

A comparison was made to a hydration or hydrolysis done by placing asecond sample of MCM-41 in water and boiling at atmospheric pressure for4 hours. In this comparison, no change or difference was observed.

EXAMPLE 2

An experiment was conducted to compare the surface density of assemblymolecules using prior liquid solution deposition as reported by Feng etal., 1997 and Liu et al, 1998 (described in Background above), and usingthe supercritical fluid solution deposition of the present invention.

Surface density was determined gravimetrically and by ²⁹Si NMR.

The surface density of the product made with the liquid solutiondeposition which included an azeotropic distillation was 5.0-5.2silanes/nm².

According to the present invention, The silica was hydrated by simplystoring it in a 100% humidity chamber and monitoring the sample's weightas a function of time, stopping at 20-25% weight gain. The MCM-41 wasadded to the sample holder along with the MPTMS (mercaptopropyltrimethoxysilane), then the system was sealed and brought up to pressureand temperature (7500 psi and 150° C.) with SCCO₂. After only 5 minutes,a monolayer with a surface density of 6.4 silanes/nm² was depositedwhich was surprisingly approximately 20% higher than achieved usingliquid phase deposition.

The spectrum of this 5-minute sample is shown in FIG. 2a. A referencepeak 200 is from TTMS tetrakis (trimethylsilyl) silane. The peak 202 isa combination of signals for Q2, Q3, and Q4 silicic acid units in thebase material. The peaks 204, 206, 208 are the internal, terminal, andisolated silanes respectively. The silane demographics of this sampleare similar to those found in monolayers prepared under atmosphericpressure and liquid solution phase conditions.

It was also found that maintaining the sample at elevated temperatureand pressure in SCCO₂ resulted in a slow but steady evolution of thesilane demographics, with a gradual decrease in the population of theterminal silane with a concomitant increase in the population of theinternal silane over 24 hours. Over this same timeframe, the signal forthe isolated silane completely disappeared, indicating an annealing ofthe “dangling” hydroxyls within the monolayer, resulting in a greaterfully bonded fraction or higher degree of siloxane cross-linking. Inthis experiment, we observed a terminal to internal silane ratio (basedupon total area under each peak) of approximately 1:4 after 24 hours(FIG. 2b). This is unexpectedly the highest degree of crosslinking in asilane based monolayer documented by ²⁹Si NMR. Ratios as a function oftime are summarized in FIG. 2c showing evolution of silane demographics.

Both the surprising surface density and the unexpected high degree ofcrosslinking are directly attributable to the use of SCCO₂ as thereaction medium.

EXAMPLE 3

An experiment was conducted as in Example 2 wherein a portion of thematerial from the 5-minute sample was re-exposed or re-introduced to thesupercritical fluid environment for an additional 30 minutes.

Results are shown in FIG. 3. The evolution or formation of the monolayercontinued in the same manner as for continuous supercritical fluidexposure.

EXAMPLE 4

An experiment was conducted to demonstrate the enhanced chemicalstability of the coating material as produced as in Example 2. Samplesof the coating material of the present invention from Example 2 wereexposed to a series of identical buffer solutions of various pH.Comparative samples of solution deposited coating material were exposedto identical buffer solutions of various pH from pH 0.5 to pH 12.5.

Results are shown in FIG. 4 wherein essentially no difference isobserved for pH less than 9, but above pH 9 up to pH 12.5, the solutiondeposited material (Regular SAMMS) exhibits a leaching or loss of sulfurto the solution indicating a degradation of the monolayer. Thesupercritical deposited material according to the present inventionshows no change in sulfur concentration above pH 9 up to pH 12.5. Thisincrease in chemical durability of the monolayer is an unexpectedresult.

EXAMPLE 5

An experiment was conducted as in Example 2 wherein Zeolite beta from(Zeolyst) in the form of beads (3 mm in diameter) was mixed withtris(methoxy)mercaptopropylsilane (TMMPS) with a zeolite to TMMPS weightratio of 0.67 and loaded into the sample container in the supercriticalreaction vessel. The system was sealed. The pressure and the temperaturewere increased to 7500 psi and 150° C. using CO₂. The zeolite wastreated under these conditions for 12 hours before the pressure wasreduced and temperature decreased to 25° C. The treated zeolite beadswere recovered after the treatment, and ground into powders for furthertreatment. Functionalized and unfunctionalized zeolites werecharacterized by X-ray diffraction (XRD). The XRD peaks recorded areconsistent with the XRD data reported for zeolite beta, but thediffraction peaks are broader and weaker. Apart from the two main peaksat 2 theta of 7.7° and 22.5°, other minor diffraction peaks are not wellresolved. The XRD data suggest that the commercial zeolite beta usedhere has a similar crystalline structure as the synthetic high purityzeolite beta, but has smaller crystallite size, or a higher degree ofdisordering. Transmission electron microscopy (TEM) images and electronenergy dispersive X-ray spectroscopy (EDX) spectra were also obtained.Because the commercial zeolite beta is not highly crystalline, thezeolite lattice fringes are not resolved in the TEM image. In the EDXspectrum, a strong Si peak, a small Al peak (from the zeolite material),and a small S peak is observed. This S peak comes from the sulfonategroup introduced into the zeolites during SCCO₂ functionalization. Fromthe EDX data the sulfonate group density in zeolite can be estimated tobe 0.87 mmol sulfonate/g zeolite. The aluminum concentration is about 2%by weight.

Typically part of the unreacted silanes and by-products would be forcedout of the inner volume of the porous substrate when the pressure wasquickly reduced in the supercritical treatment. After the supercriticaltreatment, the materials were further treated with H₂O₂/method solutionsand H₂SO₄ solutions over long periods of time during the acidifying andsulfonation procedure, which also includes many washing and rinsingsteps involving ethanol and water. It is expected that any physicallytrapped silanes or its by-products should be removed from the materialduring these treatments. This conclusion is verified by acid washingexperiments. No silane product was released, as measured by NMRexperiment, when the sulfonated zeolite was subject to extended wash in0.1 M acid solutions.

A Chemagnetics NMR spectrometer was used to obtain ²⁹Si NMR results. Itis important to recognize that relative peak intensities in ²⁹Si CP-MASare not strictly quantifiable due to differences in relaxation behavior.Therefore, we have used the Bloch decay pulse sequence (single pulseexcitation) with long recycle times (30 sec) to obtain the spectrum morerepresentative of the molecular composition of these materials. Thelarge peak at −110 ppm is from the silica support. The broad feature at−110 ppm is also indicative of the poor crystalline nature of thezeolite. Two additional peaks from −50 to −80 ppm corresponding tosiloxane groups in the functionalized zeolites are observed. Thesiloxanes peaks are much more pronounced than reported in literature,suggesting a higher surface coverage. The peak positions suggest a highdegree of crosslinking between the siloxane groups, and between siloxanegroups and the substrate. Since the small pore size excludes thepossibility of close packing of the silane groups, so most likely thesiloxanes are attached to the substrate with a tridentate, or bidentatebinding. On smooth substrates or in large pore materials, bothtridentate and bidentate bindings are not favored. This binding schemehas been reported in zeolites because of the small pore size and highcurvature.

The conversion of HEX and PYC can be easily quantified and the onlyobserved products were the mixtures of reactants and acetalized (orketalized) products. For sulfonated zeolite (Z—SO₃H), more than 60% HEXwas converted in 4 hours, and nearly complete conversion was observedover 12 hours. On the other hand, PYC, which has a large molecular sizeand cannot enter the microporosity, showed less than 8% conversion overextended reaction time with same Z—SO₃H as catalyst. These resultsindicate that the Z—SO₃H material is size selective, and that themajority of SO₃H groups are inside the microporosity and are accessibleto molecules smaller than the pore size, and inaccessible to themolecules larger than the pore size. The reaction rate with SCCO₂ Z—SO₃Hcompares favorably with similar zeolite functionalized using an in-situdeposition technique, which produced 38% conversion of HEX in 4 hoursunder the same conditions. Both HEX and PYC were also reacted over purezeolite beta (Z), and the TMMPS functionalized zeolite (Z-SH) before itwas treated with H₂O₂. Pure zeolite and Z—SH showed low catalyticactivity, and only a small fraction of either HEX and PYC was converted.It can be concluded from these results that the majority of SO₃H groupsreside inside the zeolite pore channels and act as the active center forthe reaction. It is important to note that no additional extractionprocedure was performed on the supercritically processed zeolite toremove chemically bonded sulfonated groups on the external surface ofthe zeolite. Therefore a small portion of chemically bonded sulfonatedgroups remained on the external surface, which gave rise to the residualactivity observed for PYC. However, the contribution of the externalsulfonic groups to the overall reaction is minimal. Furthermore, theactivity of the external acid groups can be selectively neutralized toachieve complete size selectivity (which has been demonstrated in ourexperiments). The external acid groups can be also removed through apost extraction treatment.

The high activity of the functionalized zeolite over the parent materialis attributed to the acidic groups introduced by the functional groups.To verify this, acid-base titration was conducted to determine thenumber of acid sites. The titration was conducted in 5.0 ml 0.1 N HCIsolution with 0.05 g suspended solid powders using a 0.1 N NaOH solutionas the titrant. The titration curve is the superposition of thetitration curve of the strong acid (HCI) and that of the surface acidgroup from the catalyst. The proton capacity (acid site density) can becalculated using standard methods. The titration experiments showed thatthe sulfonated zeolites (Z—SO₃H) have a much high proton capacity (25.7mmol/g) as compare with the native zeolite beta (3.79 mmol/g) and theunsulfonated thiol zeolite (Z—SH) (3.37 mmol/g).

For comparison, sulfonated mesoporous silica (M—SO₃H) was used tocatalyze the conversion of PYC and HEX. In this case both HEX and PYCcan easily enter the pore channel and access the catalytic SO₃H sites.Therefore low-selective conversion of both HEX and PYC were observed.

Further evidence of the size selectivity is provided when amines ofdifferent sizes are used to poison (neutralize) the acid sites. Reactionof HEX and glycol was performed over Z—SO₃H as discussed before.Triethylamine ((C₂H₅)₃N, or TEA) is added to the reaction bath after 40minutes. TEA is a small molecule and can enter the microporosity andpoison all the acid sites. The addition of TEA completely stopped thereaction. Under the same condition the addition of methyldioctylamine[(CH₃(CH₂)₇)₂NCH₃, or MDOA] instead of TEA did not have any effect onthe conversion of HEX over Z—SO₃H, because the molecular size of MDOA istoo large for it to enter the microporosity and poison the acid sites inthe internal pore channels. The addition of MDOA did effectivelyneutralize all the residual acid sites on the external surfaces ofZ—SO₃H. Zero activity was observed for PYC under these conditions. Ifthe pore size is large enough, like in mesoporous silica (M—SO₃H), MDOAis an effective poison for acid catalyzed reaction. We have shown thatthe addition of MDOA to the reaction bath of HEX over M—SO₃H completelystopped the reaction in the mesoporous materials.

CLOSURE

While a preferred embodiment of the present invention has been shown anddescribed, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its broader aspects. The appended claims are thereforeintended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

We claim:
 1. A functionalized material comprising: a a substrate, and b.a single self-assembled monolayer prepared on said substrate in asupercritical fluid, said monolayer comprising; i. a plurality ofassembly molecules, each assembly molecule having ii. an assembly atom,each assembly atom having a plurality of bonding sites greater in numberthan if said monolayer had been deposited by solution deposition.
 2. Thefunctionalized material as recited in claim 1, wherein said assemblyatom is silicon having four bonding sites and said maximum portion is40%.
 3. The functionalized material as recited in claim 2, wherein saidgreater portion is greater than 55%.
 4. The functionalized material asrecited in claim 3, wherein said greater portion is greater than orequal to about 75%.
 5. The functionalized material as recited in claim1, wherein a surface density of said plurality of assembly molecules isgreater than 5 assembly molecules per square nanometer.
 6. Thefunctionalized material as recited in claim 5, wherein said surfacedensity is about 6.5 assembly molecules per square nanometer.
 7. Afunctionalized material comprising: a. a substrate, and b. a singleself-assembled monolayer prepared on said substrate in a supercriticalfluid, said monolayer comprising; i. a plurality of assembly moleculesin an amount greater than if said monolayer had been deposited bysolution deposition, each of said assembly molecules having ii. anassembly atom, each assembly atom having a plurality of bonding sites.8. The functionalized material as recited in claim 7, wherein saidmaximum surface density is less than or equal to 5 assembly moleculesper square nanometer, and said greater density is greater than 5assembly molecules per square nanometer.
 9. The functionalized materialas recited in claim 7, wherein said plurality of assembly molecules hasa greater portion of fully bonded assembly atoms greater than a portionof fully bonded assembly atoms.
 10. The functionalized material asrecited in claim 9, wherein said greater portion of fully bonded siliconatoms is greater than 55%.
 11. The functionalized material as recited inclaim 10, wherein said greater portion of fully bonded silicon atoms isgreater than or equal to about 75%.
 12. The functionalized material asrecited in claim 7, wherein said substrate is a mesoporous material.